Liquid sensor using temperature compensation

ABSTRACT

A liquid sensor system can detect whether liquid is present at a location within a tank or other container, such as an aircraft fuel tank. The system can include a sensor that has a heated negative temperature coefficient (NTC) element and a temperature compensator element. The sensor may be located within the tank. The elements can be polarized by voltages from separate voltage sources. The voltages can be compared to detect a presence or level of liquid within the tank. Only two wires may be needed to connect the sensor with components outside the tank. The system may be able to detect liquid using less than twenty five milliamps and a sensor temperature that is less than two hundred degrees Celsius.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/533,831, titled “Two Wire Liquid Sensor Using TemperatureCompensation,” and filed Sep. 13, 2011, the entirety of which isincorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to assemblies for sensing liquidin a tank or other container and, more particularly (although notnecessarily exclusively), to sensing liquid while compensating fortemperature.

BACKGROUND

A level of liquid can be detected using a heated negative temperaturecoefficient (NTC) resistor. For example, U.S. Pat. No. 5,421,202 toPimpec relates to a sensor that includes a probe including an NTCresistor and a positive temperature coefficient resistor connected inseries. A constant current is delivered to the probe and a totalresistance of the resistors is compared with a threshold to detectwhether liquid is present at a location in a tank. Some liquid leveldetection systems use a self-heating thermistor probe (e.g. U.S. Pat.No. 4,276,536). U.S. Pat. No. 4,416,153 to Williams is another exampleof a method that includes comparing voltage levels to thresholds.

A dual thermistor bead sensor can be used to detect a level of liquid.For example, U.S. Pat. No. 6,662,650 to Durkee, et al. relates toconducting different currents through different thermistor beads,measuring voltages across the thermistor beads, generating signals fromthe voltage measurements, modifying one of the signals by an offset andgain, and detecting a dry/wet state of the sensor based on the modifiedsignal and another generated signal. U.S. Pat. No. 6,758,084 to Hall isanother example.

Liquid level detection methods as in the examples are useful in limitedtemperature ranges. When the temperature range is large, and theenvironment is potentially explosive, present solutions may not be fullysatisfactory.

Other methods and apparatus compensate a thermistor bead fortemperature. U.S. Pat. No. 6,822,460 to Pelkey relates to detecting adry/wet state of a thermistor bead using temperature compensation. Athermistor bead is disposed at a height in a fuel tank. A temperaturesensor is disposed in the fuel tank and can measure a temperature inproximity to the thermistor bead. One circuit conducts a constant biascurrent of less than thirty milliamps into the fuel tank and through thethermistor bead. A second circuit generates a reference voltage inproportion to the measured temperature of the sensor. A third circuitmeasures a voltage across the thermistor bead in response to the biascurrent, and can detect the dry/wet state of the thermistor bead basedon the measured and reference voltages.

Methods using temperature compensation, however, may require severalwires and sophisticated electronics, such as current sources providingpolarization current.

In some environments in which a tank is located—such as in anaircraft—weight is a concern, as is the presence of several wires. It isalso desirable for tank components to be simple for reliability,compatibility, accessibility purposes. A system using an NTC polarizedby current, in contrast, may use a current source that is relativelycomplex to design.

In connection with aircraft, the Federal Aviation Administration (FAA)recently defined an acceptable maximum current and maximum temperaturefor in aircraft tank applications. These values are twenty fivemilliamps and two hundred degrees Celsius, respectively. Liquid sensorsystems are desirable that can operate within these requirements andthat involve simple-to-design components and fewer wires.

SUMMARY

Certain aspects of the present invention are directed to systemincluding a sensor for detecting whether liquid is present at a locationin a tank, such as an aircraft fuel tank, using temperaturecompensation. The sensor can include a negative temperature coefficient(NTC) element and a temperature compensation element that are polarizedby voltages from different voltage sources. The number and complexity ofthe components needed to implement the system may be reduced and FAArequirements can be met.

One aspect relates to a system that includes a sensor and comparisoncircuitry. The sensor can be in a tank that can contain liquid. Thesensor can include an NTC element and a temperature compensator element.The NTC element can be polarized by a first voltage source through afirst resistor. The temperature compensator element can be polarized bya second voltage source through a second resistor. The comparisoncircuitry can be outside the tank. The comparison circuitry candetermine a dry/wet state within the tank by comparing voltages acrossthe NTC element and the temperature compensator element.

Another aspect relates to a liquid sensor system that includes a sensor,a modulator, a demodulator, and comparison circuitry. The sensor can bewithin a tank. The modulator, demodulator, and comparison circuitry canbe outside the tank. The sensor can include a heated NTC element and atemperature compensator element. The modulator can modulate voltages forpolarizing the heated NTC element and the temperature compensatorelement. The demodulator can extract the voltages across the heated NTCelement and the temperature compensator element. The comparisoncircuitry can determine a dry/wet state within the tank by comparing thevoltages across the heated NTC element and the temperature compensatorelement.

Another aspect relates to a system that includes a sensor, polarizationcircuitry, and measurement circuitry. The sensor is within a tankadapted to contain liquid. The sensor can include a heated NTC elementand a temperature compensator element. The polarization circuitry isoutside the tank and connected to the sensor by only two wires. Thepolarization circuitry can provide polarization voltages to the sensorfrom separate voltage sources. The measurement circuitry is outside thetank and can determine a dry/wet state within the tank by comparingvoltages across the heated NTC element and the temperature compensatorelement.

These illustrative aspects and features are mentioned not to limit ordefine the invention, but to provide examples to aid understanding ofthe inventive concepts disclosed in this application. Other aspects,advantages, and features of the present invention will become apparentafter review of the entire application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic diagram of a three-wire liquid sensor system withtemperature compensation according to one aspect of the presentinvention.

FIG. 2 is a chart of voltages with respect to temperature according toone aspect of the present invention.

FIG. 3 is a schematic diagram of a liquid sensor system using signalmodulation according to one aspect of the present invention.

FIG. 4 depicts an example of a logic modulation signal according to oneaspect of the present invention.

FIG. 5A is a schematic of a negative envelope detector according to oneaspect of the present invention.

FIG. 5B is a schematic of a positive envelope detector according to oneaspect of the present invention.

FIG. 6 is a block diagram of a two-wire liquid sensor system accordingto one aspect of the present invention.

FIG. 7 is a schematic of a two-wire liquid sensor system according toone aspect of the present invention.

FIG. 8 is a schematic of a sensor according to one aspect of the presentinvention.

DETAILED DESCRIPTION

Certain aspects and features relate to a liquid sensor system fordetecting whether liquid is present at a location within a tank or othercontainer, such as an aircraft fuel tank. The system can include asensor, or probe, that includes a negative temperature coefficient (NTC)element and a temperature compensator element. The sensor may be locatedwithin the tank. The elements can be polarized by voltages from separatevoltage sources. The voltages can be compared to detect a presence orlevel of liquid within the tank. An example of the sensor is athermistor.

In some aspects, the NTC element is a heated NTC resistor. The systemcan include modulation circuitry for modulating voltages to polarize theelements. Some examples also include multiplexing circuitry to multiplexthe modulated voltages on a wire. The sensor may include or beassociated with a demultiplexer that can extract the voltages for theelements. The sensor and demultiplexer can be connected by two wires tosystem components outside the tank, reducing the number of wires neededto implement the liquid sensor system.

The temperature compensator element may not be a heated NTC. Thetemperature compensator element may have a resistive value that isindependent of fluid (e.g. air or fuel) contact such that the resistancevalue is only a function of the temperature such that there is noheating. The variation due to fluid may be negligible.

Current through the system according to various aspects can be less thantwenty five milliamps and the sensor temperature can be less than twohundred degrees Celsius.

Certain aspects simplify the calculation of electric parameters byhaving a fixed current, with resistance and voltage varying. In otheraspects, a voltage generator may be used as a voltage source to polarizethe NTC element through a resistor with resistance, voltage, and currentbeing variable. A voltage generator may be more straightforward toimplement than a current source in previous systems, and complexity canbe compensated by present simulation tools.

These illustrative aspects and examples are given to introduce thereader to the general subject matter discussed here and are not intendedto limit the scope of the disclosed concepts. The following sectionsdescribe various additional features and examples with reference to thedrawings, but, like the illustrative aspects, should not be used tolimit the present invention.

FIG. 1 depicts a liquid sensor system according to one aspect. Theliquid sensor system includes a sensor 102 within a tank and electronicmeasurement circuitry 104 outside, or external to, the tank. A dashedvertical line in FIG. 1 represents a divider between components withinthe tank and components outside the tank.

The sensor 102 includes two elements R1, R2. Element R1 may be a heatedNTC thermistor having a value that is dependent on temperature and thethermal properties of fluid (e.g. air or fuel) within the tank. In otheraspects, element R1 is an NTC thermistor. Element R2 may be temperaturecompensator element, such as a Resistance Temperature Dependent (RTD)thermistor having a resistance that changes as a function oftemperature.

The electronic measurement circuitry 104 can include voltage sources V1,V2, resistors R3, R4, and a comparator 106. Voltage sources V1, V2 maybe direct current (DC) voltage sources. In some aspects, the voltagesources V1, V2 are separate voltage sources that can provide polarizingvoltages. In other aspects, the voltage sources V1, V2 are integratedinto a single device that can output separate voltages. The comparator106 may be any device that can compare two voltages and output a signalor value that can represent a wet/dry state within the tank.

Elements R1, R2 can be polarized by voltages from separate voltagesources V2, V1 through separate resistors R3, R4. Element R1 can bepolarized by a voltage from voltage source V1 through resistor R3.Element R2 can be polarized by a voltage from voltage source V2 throughresistor R4.

The voltages across elements R1, R2 can be compared by the comparator tooutput a value representing the wet/dry state within the tank. Forexample, the voltage across element R1 may be a measurement voltage(labeled in FIG. 1 as “V_measure”) and the voltage across element R2 maybe a reference voltage (labeled in FIG. 1 as “V_Reference”).

FIG. 2 depicts examples of voltages at V_measure and V_Reference withrespect to temperature. Line 202 represents voltage at V_measure whenelement R1 is substantially in fuel within the tank. Line 204 representsvoltage at V_measure when element R1 is substantially in air within thetank. Line 206 represents V_Reference. Systems according to some aspectscan be configured such that line 206, representing V_Reference, isbetween lines 202, 204. For example, tolerances of components can betaken into account such that the parameters of elements R1, R2 can beselected to cause line 206 to be between lines 202, 204.

In other aspects, polarizing voltages can be modulated. FIG. 3 depictsanother aspect of a liquid sensor system that includes modulationcircuitry 302 in electronic measurement circuitry 304. The electronicmeasurement circuitry also includes voltage sources V2, V1, resistorsR3, R4, comparator 106, negative demodulation device 308, positivedemodulation device 310, and multiplier devices 312, 314. The modulationcircuitry includes multiplier devices 316, 318 and inverter 320.

A multiplier device may include circuitry that can multiply two or moreinputs and provide one output that is a product of the inputs. Anexample of a multiplier device is a mixer. In some aspects, negativedemodulation device 308 and positive demodulation device 310 areenvelope detectors.

Voltage from voltage source V1 can be modulated in a positive waveformusing a modulation technique. An example of a modulation technique isamplitude modulation, but other techniques, such as frequencymodulation, can also be used. In multiplier device 316, voltage fromvoltage source V1 can be multiplied with a logic modulation signal thatmay be provided by, or received from, any source that can provide amodulation signal. The output of the multiplier device 316 may be apolarization signal for element R1. The polarization signal canrepresent the voltage from voltage source V1.

Voltage from voltage source V2 can be modulated in a negative waveform.Voltage from voltage source V2 can be multiplied by multiplier device314 with an inverting value, such as negative one. The output ofmultiplier device 314 may be the inverse of the voltage from voltagesource V2. The inverse of the voltage from voltage source V2 can bemultiplied by multiplier device 318 with an inverse of the logicmodulation signal from inverter 320. The output of the multiplier device318 may be a polarization signal for element R2 that can represent thevoltage from voltage source V2.

In other aspects, voltage from voltage source V1 is modulated in anegative waveform and voltage from voltage source V2 is modulated in apositive waveform.

Voltages represented by the polarization signals can be demodulatedusing the negative demodulation device 308 and the positive demodulationdevice 310. The output of the negative demodulation device 308 can bemultiplied by multiplier device 312 with an inverting value, such asnegative one, to invert the voltage back. The output of the multiplierdevice 312 can be compared to the output of the positive demodulationdevice 310 by comparator 106. The output of the comparator 106 canrepresent the wet/dry state within the tank.

Although depicted as separate devices, negative demodulation device 308and positive demodulation device 310 may be circuitry in a single devicein other aspects.

FIG. 4 depicts an example of a logic modulation signal according to oneaspect. Two parameters of the logic modulation signal include the periodand the modulation ratio between reference and measured values. Theperiod may define the time to refresh data. The ratio may define timesharing between the two signals. In some aspects, a ratio in the rangeof ninety to less than one hundred percent provides a suitable balance.For example, measured may be ninety-five percent and the reference fivepercent.

FIGS. 5A and 5B depict examples of circuitry for negative demodulationdevice 308 and positive demodulation device 310, respectively. Thecircuitry includes a resistor, a capacitor, and a diode. The diodes maybe any unidirectional components. Diodes may be configured in oppositedirections. The circuitry in FIGS. 5A and 5B can split the positive andnegative voltage waveforms into two independent signals. Although twoenvelope detectors, one in positive and the other in negative, aredepicted, other solutions may be used, with or without coherentdemodulation.

A liquid sensor system according to some aspects may only use two wiresconnecting a sensor and/or other components within a tank withcomponents outside the tank. FIG. 6 depicts a two-wire liquid sensorsystem according to one aspect. The system includes a sensor unit 402, apolarization circuit 404, a measure circuit 406, and a monitoringcircuit 408. The sensor unit 402 can be within the tank and the othercomponents can be outside the tank. The sensor unit 402 can connect tothe other components using two wires 412, 414. For example, wire 412 mayallow voltage or polarization signal to be provided to the sensor unit402 and wire 414 may be a ground wire.

The polarization circuit 404 can provide voltages from voltage sourcesin the polarization circuit 404. The measure circuit can comparevoltages across elements in the sensor unit 402 to output arepresentation of a wet/dry date within the tank. The monitoring circuit408 can be an independent measurement circuit that can also measure thevoltages to calculate a failure status. For example, in the case of ashort circuit or open circuit of the sensor unit 402, the monitoringcircuit 408 can detect a failure of the sensor unit 402 and output astatus indicating a failure.

FIG. 7 schematically depicts a two-wire liquid sensor system accordingto one aspect. The system includes the sensor 102 and a demultiplexer502 within the tank. The system also includes electronic measurementcircuitry 504 outside of the tank. The electronic measurement circuitry504 includes the components in electronic measurement circuitry 304 ofFIG. 3 and a multiplexer 506 that can receive a logic multiplexingsignal. Two wires 508, 510 connect the sensor 102 and demultiplexer 502with components outside of the tank.

In the example depicted in FIG. 7, polarization signals from themodulation circuitry 302 through resistors R3, R4 are multiplexed bymultiplexer 506 using time-division multiplexing into a multiplexedsignal that includes representations of voltages from voltage source V1and voltage source V2. Wire 508 can transport the multiplexed signal tothe demultiplexer 502. The demultiplexer 502 can recover thepolarization signals and provide each polarization signal to element R1or element R2, as the case may be. Elements R1, R2 may be in series withthe demultiplexer 502.

Resistors R3, R4 may limit current to twenty five milliamps or less inthe case of a short circuit of the sensor. Modulation and multiplexingmay allow the use of relatively simple electronic circuits fordemultiplexing and demodulating.

FIG. 8 schematically depicts a sensor and part of a demultiplexeraccording to one aspect. The sensor includes an NTC element (labeled as“NTC1”) and a temperature compensator element (labeled as“Compensator”). The sensor elements are connected together byunidirectional components, such as diodes D1, D2 in opposite directions.The diodes D1, D2 may be part of the demultiplexer, such asdemultiplexer 502 in FIG. 7, or separate components. The arrangement ofthe diodes D1, D2 in opposite directions can allow the operating of theNTC element for positive modulation and the temperature compensatorelement for negative modulation, or vice versa according to theconfiguration of the system.

The foregoing description of the aspects, including illustrated aspects,of the invention has been presented only for the purpose of illustrationand description and is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Numerous modifications,adaptations, and uses thereof will be apparent to those skilled in theart without departing from the scope of this invention.

What is claimed is:
 1. A system for sensing liquid in a tank comprising:a sensor adapted to be within a tank configured to contain liquid, thesensor comprising: a negative temperature coefficient (NTC) elementconfigured for being polarized by a first voltage source through a firstresistor; and a temperature compensator element configured for beingpolarized by a second voltage source through a second resistor; andcomparison circuitry adapted to be outside the tank, wherein thecomparison circuitry is configured for determining a dry/wet statewithin the tank by comparing voltages across the NTC element and thetemperature compensator element while maintaining the voltage across theNTC element and the first resistor and the voltage across thetemperature compensator element and the second resistor.
 2. The systemof claim 1, wherein the NTC element is a heated NTC element, wherein theheated NTC element and the temperature compensator element areconfigured for being polarized using a polarization current that is lessthan 25 milliamps, wherein a temperature of the heated NTC element isless than 200 degrees Celsius independent of a temperature of anenvironment in which the tank is located.
 3. The system of claim 1,further comprising the first resistor and the second resistor, whereinin response to a short circuit, the first resistor and the secondresistor are configured to limit current to less than 25 milliamps totank wiring.
 4. The system of claim 1, wherein the first voltage sourceand the second voltage source are direct current voltage sources.
 5. Thesystem of claim 1, wherein the temperature compensator element is anon-heated element and is configured to have a resistive value that is afunction of temperature and is independent of fluid contact with thetemperature compensator element.
 6. The system of claim 1, furthercomprising: a modulator configured for amplitude modulating voltagesfrom the first voltage source and the second voltage source into a firstpolarization signal for the NTC element and a second polarization signalfor the temperature compensator element; and a demodulator configuredfor amplitude demodulating and extracting the voltages across the NTCelement and the temperature compensator element from the firstpolarization signal and the second polarization signal.
 7. The system ofclaim 6, further comprising: a multiplexer adapted to be outside thetank, the multiplexer being configured for multiplexing the firstpolarization signal and the second polarization signal into amultiplexed signal; and a demultiplexer adapted to be within the tank,the demultiplexer being configured for recovering the first polarizationsignal and the second polarization signal from the multiplexed signal,for providing the first polarization signal to the NTC element, and forproviding the second polarization signal to the temperature compensatorelement.
 8. The system of claim 7, wherein the demultiplexer isconfigured to be connected to components outside the tank by only twowires.
 9. The system of claim 7, wherein the NTC element and thetemperature compensator element are thermistors, the demultiplexer beingin series with the NTC element and the temperature compensator element,wherein the demultiplexer comprises unidirectional components inopposition on each branch in the demultiplexer.
 10. The system of claim9, wherein the unidirectional components comprise diodes.
 11. The systemof claim 6, wherein the demodulator comprises an envelope detectorconfigured for coherent or non-coherent demodulation.
 12. The system ofclaim 6, wherein the modulator is an amplitude modulation modulator. 13.The system of claim 1, further comprising: monitoring circuitry outsidethe tank, the monitoring circuitry being configured for, independent ofthe comparison circuitry, measuring the voltages across the NTC elementand the temperature compensator element and detecting a failure of thesensor.
 14. An aircraft fuel tank comprising: a tank for holding fuelfor an aircraft; and a system for sensing liquid in the tank, the systemcomprising: a sensor adapted to be within the tank, the sensorcomprising: a negative temperature coefficient (NTC) element configuredfor being polarized by a first voltage source through a first resistor;and a temperature compensator element configured for being polarized bya second voltage source through a second resistor; and comparisoncircuitry adapted to be outside the tank, the comparison circuitry beingconfigured for determining a dry/wet state within the tank by comparingvoltages across the NTC element and the temperature compensator elementwhile maintaining the voltage across the NTC element and the firstresistor and the voltage across the temperature compensator element andthe second resistor.
 15. The aircraft fuel tank of claim 14, wherein theNTC element is heated NTC element, wherein the heated NTC element andthe temperature compensator element are configured for being polarizedusing a polarization current that is less than 25 milliamps, wherein atemperature of the heated NTC element is less than 200 degrees Celsiusindependent of a temperature of an environment in which the tank islocated.
 16. The aircraft fuel tank of claim 14, further comprising thefirst resistor and the second resistor, wherein in response to a shortcircuit, the first resistor and the second resistor are configured tolimit current to less than 25 milliamps to tank wiring.
 17. The aircraftfuel tank of claim 14, wherein the first voltage source and the secondvoltage source are direct current voltage sources.
 18. The aircraft fueltank of claim 14, wherein the temperature compensator element is anon-heated element and is configured to have a resistive value that is afunction of temperature and is independent of fluid contact with thetemperature compensator element.
 19. The aircraft fuel tank of claim 14,further comprising: a modulator configured for amplitude modulatingvoltages from the first voltage source and the second voltage sourceinto a first polarization signal for the NTC element and a secondpolarization signal for the temperature compensator element; ademodulator configured for amplitude demodulating and extracting thevoltages across the NTC element and the temperature compensator elementfrom the first polarization signal and the second polarization signal; amultiplexer adapted to be outside the tank, the multiplexer beingconfigured for multiplexing the first polarization signal and the secondpolarization signal into a multiplexed signal; and a demultiplexeradapted to be within the tank, the demultiplexer couplable to componentsoutside the tank by only two wires and being configured for recoveringthe first polarization signal and the second polarization signal fromthe multiplexed signal, for providing the first polarization signal tothe NTC element, and for providing the second polarization signal to thetemperature compensator element.